Fluid inclusion microthermometry to calibrate vitrinite reflectance (between 50 and 270 °C), illite Kübler-Index data and the diagenesis/anchizone boundary in the external part of the Central Alps

Abstract Fluid inclusion composition and homogenization temperatures in fibre and prismatic quartz from 45 Alpine fissures formed synkinematically during orogenic metamorphism in the external part of the Central Alps were compared with organic matter maturation determined by vitrinite reflectance %R r (random)/%R max of the host rocks. Fluid immiscibility led to both, coexisting water bearing higher hydrocarbon- (HHC) or methane-rich fluid inclusions and HHC or methane bearing water-rich fluid inclusions. The homogenization temperatures of the latter reveal inclusion formation temperatures. Earliest fibre and prismatic quartz in diagenetic and low-grade anchimetamorphic rocks crystallized more or less close to maximum temperatures. Thus, the homogenization temperatures of hydrocarbon saturated water-rich fluid inclusions are used as geothermometer between ~ 50 and 270 ± 5 °C. The process of vitrinite maturation is irreversible and the level of organic maturity is basically the product of temperature and time at moderate pressures (~ 1 to ≥ 3 kbar). Based on the close interaction of solid (vitrinite), liquid and volatile organic matter (i.e. HHC, CH 4 ) and water, fluid inclusion homogenisation temperatures are used to calibrate vitrinite reflectance (VR) measurements in the temperature range from ~ 50 to 270 ± 5 °C. The following relationship between fluid inclusion homogenization temperatures, VR and fluid composition for the external part of the Central Alps has been established: 200 ± 5 ° C VR = 2.4 ± 0.2 % R max : Transition of higher hydrocarbons wet gas to methane with ≤ 1 mole % of wet gas 270 ± 5 ° C VR = 5.7 ± 0 . 7 % R max : End of the methane stability due to red ‐ ox reactions . Applying fluid inclusion data and the VR versus illite Kubler-Index (KI) correlation the paleo-geothermal gradient is determined. In different structural units of the external part of the Central Alps the conditions vary between 45 and ≤ 20 °C/km. At these normal to slightly enhanced orogenic geothermal gradients and at a heat flow = 65 ± 10 mW/m 2 the KI 0.42 Δ°2θ of the diagenesis/anchizone boundary is fixed at: 240 ± 15 ° C and VR = 4.0 ± 0.7 % R max It is envisaged that this temperature limit may be indicative for external orogenic settings at normal to slightly hyperthermal heat flow. This will help to a better understanding of correlating organic matter versus clay mineral indices to determine geothermal settings for basic research and exploration.